Multimode fibers are successfully used in high-speed data networks together with high-speed sources that typically use transversally multimode vertical cavity surface emitting lasers, more simply called VCSELs. However, multimode fibers are affected by intermodal dispersion, which results from the fact that, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information but travelling with different propagation velocities. Modal dispersion is expressed in terms of Differential Mode Delay (DMD), which is a measure of the difference in pulse delay between the fastest and slowest modes traversing the multimode fiber.
In order to minimize modal dispersion, the multimode optical fibers used in data communications generally comprise a core showing a refractive index that decreases progressively going from the center of the fiber to its junction with a cladding. In general, the index profile is given by a relationship known as the “α profile,” as follows:
            n      ⁡              (        r        )              =                            n          0                ⁢                              1            -                          2              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                                                (                                      r                    a                                    )                                α                                                    ⁢                                  ⁢                  for                ⁢                                  ⁢        r            ≤      a        ,where:                n0 is a refractive index on an optical axis of a fiber;        r is a distance from said optical axis;        a is a radius of the core of said fiber;        Δ is a non-dimensional parameter, indicative of an index difference between the core and a cladding of the fiber; and        α is a non-dimensional parameter, indicative of a shape of the index profile.        
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is thus possible to theoretically obtain a group velocity, which is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength. However, an optimum value of the parameter α is valid for a particular wavelength only. Furthermore, the exact parameter value α, as well as the actual shape of the refractive index profile, are difficult to control during manufacture of the multimode fiber.
Enabled by VCSEL technology, high-speed multimode optical fibers, such as OM4 fibers (which are laser-optimized, high bandwidth 50 μm multimode fibers, standardized by the International Standardization Organization in document ISO/IEC 11801, as well as in TIA/EIA 492AAAD standard), have proved to be the medium of choice for high data rate communications, delivering reliable and cost-effective 10 to 100 Gbps solutions. The combination of Wide-Band (WB) multimode fibers with longer-wavelengths VCSELs for Coarse Wavelength Division Multiplexing (CWDM) is an interesting option to be considered in order to meet the future increase of demand.
However, the modal bandwidth of OM4 fibers until now has only been achieved over a narrow wavelength range (typically 850 nm+/−10 nm). The feasibility of Wide-Band (WB) multimode fibers satisfying OM4 performance requirements over a broader wavelength range is a challenge to overcome for next generation multimode systems.
The OM4 fiber performance is usually defined by an Effective Modal Bandwidth (EMB) assessment at a given wavelength λ0. For instance, OM4 fibers should exhibit EMB greater than 4,700 MHz-km at a wavelength of 850 nm+/−1 nm. The achievement of such high EMB values requires an extremely accurate control of refractive index profiles of multimode fibers. Up to now, traditional manufacturing processes cannot guarantee such high EMB, and generally it is hard to accurately predict the EMB values from refractive index profile measurements on core rod or cane, especially when high EMB (typically larger than 2,000 MHz-km) is expected, meaning the fiber refractive index profile is close to the optimal profile. As a matter of fact, EMB is directly assessed on fibers.
The Effective Modal Bandwidth (EMB) is assessed by a measurement of the delay due to the modal dispersion, known under the acronym DMD for “Dispersion Modal Delay.” It consists of recording pulse responses of the multimode fiber for single-mode launches that radially scan the core. It provides a DMD plot that is then post-processed in order to assess the minimal EMB a fiber can deliver. The DMD measurement procedure has been the subject of standardization (IEC 60793-1-49 and FOTP-220) and is also specified in Telecommunications Industry Association Document no. TIA-455-220-A. The DMD metric, also called DMD value, is expressed in units of picoseconds per meter (ps/m). It assesses the delay between the fastest and the slowest pulses considering a collection of offset launches normalized by fiber length. It basically assesses a modal dispersion. Low DMD value (i.e., low modal dispersion as measured by DMD) generally results in higher EMB.
Basically, a DMD graphical representation is obtained by injecting a light pulse having a given wavelength λ0 at the center of the fiber and by measuring the pulse delay after a given fiber length L, the introduction of the light pulse of given wavelength λ0 being radially offset to cover the entire core of the multimode fiber. Individual measurements are thus repeated at different radial offset values so as to provide a cartography of the modal dispersion of the examined multimode fiber. The results of these DMD measurements are then post-processed to determine an effective transfer function of the optical fiber, from which a value of EMB may be determined.
Nowadays, all multimode fiber manufacturers perform DMD measurements and EMB assessments at a single wavelength only (typically at 850 nm+/−1 nm for OM4 qualification and +/−10 nm for OM3 qualification) of their whole production.
With the advent of new multimode fiber applications requiring high EMB over a wide operating window, one of the main concerns of the multimode fiber manufacturers is to have the ability to easily assess the EMB over a wide wavelength range.
Using the aforesaid classical measurement procedure (comprising a series of DMD measurements and an EMB assessment) to determine the optical fiber's EMB over a range of wavelengths (i.e., at a plurality of wavelengths) would require performing as many measurement procedures as there are wavelengths in the wavelength range of interest.
Making distinct independent DMD and EMB measurements to determine the optical fiber's EMB at multiple wavelengths greatly leads to an increase of the measurement time and thus the cost of measuring and producing the wide-band multimode fiber. Such a solution would notably require implementation of several light sources, each emitting in a distinct wavelength and several corresponding detectors, which would represent a complex and costly operation.
Therefore, there remains a need for a simple and low-cost method for assessing the performance in terms of Effective Modal Bandwidth of a wide-band multimode optical fiber over multiple wavelengths.